This invention relates to a fixture for electro-chemical machining for the production of long, curved cavities in a metallic component acting as an anode, with a working electrode mounted on a power-actuated guide linkage performing a feed movement in the Z-direction in an electrolyte, and in particular, for the manufacture of hollow fan blades for aircraft engines.
Forming processes and corresponding fixtures which are based on the principle of electro-chemical removal of material are generally known. In an electrochemical machining process disclosed in Specification DE 29 03 873, for example, an electrolytic solution flows through the working gap between the two electrodes, i.e. the tool and the workpiece, with material being removed from the workpiece according to the principle of the electrochemical cell and carried off in the form of metal hydroxide in the saturated, continuously outflowing electrolytic solution. With gradual feed of one of the electrodes, a channel with a cross-section conforming to the shape of the tool plus a working gap is produced in the workpiece, for example. However, the span of application of electrochemical material removal (electro-chemical machining) for the production of long cavities—for example in the manufacture of hollow blades for aircraft engines from solid material—is limited in that the tool acting as working electrode is unable to follow the complex—curved or twisted—shape of the channel corresponding to the respective workpiece contour and, further, the shape and size of the cross-section of the channel produced cannot be adapted to a workpiece wall whose thickness alters along the channel, which would be requisite for variable material removal.
In the case of fan blades of aircraft engines, it is important that, for example for weight reduction, cavities in the form of channels are produced in the interior of the blades which follow the complex, curved and twisted, blade profile and, as regards the size of the channel cross-section, adapt to the altering blade thickness, in order to remove a maximum of material over a great blade length and produce a lightweight, but robust blade from a prefabricated blank made of solid material, for example titanium. The blades for the fan of an aircraft engine must, on the one hand, be strong enough to withstand the forces arising from natural frequencies, vibration amplitude, centrifugal forces and bird strikes, and, on the other hand, satisfy the requirements for light weight and manufacturing costs lower than the state of the art.
The known methods for the manufacture of hollow fan blades in titanium are based on expensive forming and jointing processes to provide so-called built structures, as disclosed in Specifications U.S. Pat. Nos. 5,692,881 or 6,033,186, for example. For weight reduction, other known blade designs comprise a combination of titanium and carbon fiber composite, with the fiber composite being used on the pressure side of the blades only (U.S. Pat. No. 5,655,883) or a fiber composite core being provided with a metallic shielding (U.S. Pat. No. 5,876,651, DE 196 27 680). These designs of fan blades are also disadvantageous in that they incur high manufacturing investment and cost.
Moreover, as regards the fan blades of smaller or medium-sized aircraft engines, the known lightweight blade designs are even less suitable since the weight saving achievable with these designs is poor in relation to the manufacturing costs and, further, their bird strike strength fails to satisfy minimum requirements.
It has also been suggested to produce, by known electro-erosive methods and fixtures, one or more cavities only in the blade tip of a solid titanium blade to lower the weight of the blank at least in the tip area, thereby helping reduce the centrifugal forces acting on the blade root. The reduction in blade weight obtained by this approach is, however, relatively small.